docs(web_workers/shared/message_bus.ts): correct typo 'messsage' to 'message'

Corrects the typo 'messsage' to 'message' on line 35.
Closes #4085

modules/angular2/src/web_workers/shared/message_bus.ts

@@ -32,7 +32,7 @@ export /* abstract (with TS 1.6) */ class MessageBus implements MessageBusSource
   attachToZone(zone: NgZone): void { throw _abstract(); }
 
   /**
-   * Returns an {@link EventEmitter} that emits every time a messsage
+   * Returns an {@link EventEmitter} that emits every time a message
    * is received on the given channel.
    */
   from(channel: string): EventEmitter { throw _abstract(); }